Wave transmission network



Dec. 9, 1947., A. G. FOX

WAVE TRANSMISSION NETWORK Filed July 30, 1942 3 Sheets-Sheet 1 INVENTORA .6. FOX

4 TTORNE V Dec. 9, 1947. AIG. FOX 2,432,093

WAVE TRANSMISSION NETWORK Filed July 50, 1942 3 Sheets-Sheet 2 [N VE NTOR AG. FOX BY A TTORNEV Dec. 9, 1947. A. G. FOX

WAVE 'rmnsmssxou umwoax Filed July 30, 1942 3 Sheets-Sheet 3 INVENTOR A.6. FOX

A TTORNEV Patented Dec. 9, 1947 UNITED STATES PATENT OFFICE WAVETRANSMISSION NETWORK Application July 30, 1942, Serial No. 452,851

. 39 Claims. 1

This invention relates to wave transmission networks and moreparticularly to frequency selective networks for use in the transmissionof guided electromagnetic waves.

An object of the invention is to transmit freely a band of guidedelectromagnetic waves while ef- Ifaectively blocking waves fallingoutside of the and.

Another object is to separate electromagnetic waves into individualchannels on a frequency basis.

A further object is to connect without appreciable reflection two waveguides which differ in characteristic impedance.

Another object is to provide simple series resonant impedance branchesand simple parallel resonant impedance branches for use in wave guides.

A further object of the invention is to provide variable capacitors andvariable inductors for use in wave guides.

A uniform metallic sheath with or without a dielectric filler will serveas a guide for suitable electromagnetic waves. In cross section .thesheath may be circular, rectangular, or of other shape. For allfrequencies above a minimum, known as the cut-off frequency, the guideacts like a transmission line and has a specific propagation constantand characteristic impedance. For any particular frequency there are aninfinite number of cross-sectional sizes and shapes of guide which willhave the same characteristic impedance.

Shunt reactive elements are obtained by placing partial obstructionsacross the wave guide. In accordance with the present invention, shuntreactive elements for dominant transverse electric waves are obtained byusing a transverse metal partition having a slit therein which extendssubstantiallyirom one side to the other. If the slit is perpendicular tothe direction of polarization of the electric field the element isprimarily capacitive, and if parallel with the field the element isprimarily inductive. If the slot is replaced by a centrally locatedsquare or circular opening, the reactance will still be dominantlyinductive.

For a rectangular guide a rectangular opening in the partition may beproportioned to provide par allel resonance, that is, a high shuntimpedance. The resonance may be sharpened by providing inwardlyextending projections on opposite sides of the rectangular opening. Aseries resonance may be provided by making the slot sufficiently narrow.A wider opening may be used if the opposed edges of the slot are madethicker, or if the two halves of the partition are made to overlap.

A variable capacitor is provided by a pair of opposed diametral screwsextending through the guide wall in the direction of the field. Avariable inductor is provided by a, strip of spring metal which isplaced inside the guide and normally extends around the inner surface.Adjustment is made by means of a pair of opposed diametral screwsperpendicular to the field which force the strip away from the wall asthey are screwed in.

In accordance with the invention, the reactive elements just describedare combined with sections of a wave guide to provide transmissionnetworks such, for example, as wave filters and transformers. A simplefilter is formed by inserting two apertured partitions in a. guide at aproperly chosen distance apart. A variable reactor placed at anintermediate point facilitates the adjustment of the characteristics ofthe filter. By proper adjustment of the apertures the filter may be madean impedance transforming network for connecting two guides of differentcharacteristic impedance.

An impedance transforming bend is disclosed in which refiectionlesstransmission is obtained by the addition of a metallic flap which isused to provide the required aperture at'the junction of the two guides.There is also shown a transformer for connecting an air-filled tubularguide to a guide having a dielectric core, in which the core extendsinto the end of the air-filled guide. A quarter-wave transformer isdisclosed in which the capacitative reactance at the points of junctionis neutralized by the addition of metallic flaps to constrict theapertures.

Filters with improved transmission characteristics are formed byconnecting two or more chambers in tandem. The chambers may be tuned bymeans of variable reactors.

Band suppression filters with improved transmission characteristics areformed by providing two or more branch chambers spaced along the waveguide. Alternatively, a plurality of coupled chambers may be used in asingle branch. For certain effects a variable reactor may be connectedin the side branch at some point between the first apertured partitionand the point of juncture between the branch and the main guide.

Also, in accordance with the invention, it is shown how a plurality ofband-pass filters opening into a common wave guide may be arranged sothat each filter will select a certain desired band of frequencieswithout adversely afiecting transmission in the other channels.

The nature of the invention will be more fully understood from thefollowing detailed description and by reference to the accompanyingdrawings, in which like reference characters refer to similar parts andin which: 2

Figs. 1, 2 and 3 are perspective views of wave ing a high shuntimpedance, in a rectangular wave guide I. The partition I Iv has asymmetrically placed aperture l2 having a height V in a directionparallel to the electric field E and a guides having therein partitionswith apertures which provide reactive elements;

Fig. 11 shows an impedance transforming bendfor a wave guide;

Figs. 12, 13 and 14 show transformers for connecting an air-filled waveguide to a guide having a solid dielectric core;

Fig. 15 shows a neutralized quarter-wave transformer;

Fig. 16 shows a two-chamber filter with variable capacitive reactors;

Fig. 17 shows a two-chamber filter with variable inductive reactors;

Fig. 18 shows a three-chamber filter;

Fig. 19 shows a band suppression filter comprising three branchchambers;

Fig. 20 shows a band suppression filter comprising two coupled chambersin a single branch;

Fig. 21 shows a side branch with a. variable reactor; and

Fig. 22 shows five band-pass filters branching from a common wave guide.

Taking up the figures in more detail, Fig. 1 is a perspective view of asection of a metallic wave guide I, in the form of a rectangular sheath,which has been cross-sectioned just ahead of a transverse, metallicpartition comprising an upper portion 2 and a lower portion 3 with anaperture 4 therebetween extending from one side of the guide to theother. If the guide I is carrying dominant transverse electric waveswith the electric field E polarized in a. direction perpendicular to thelength of the aperture 4, as indicated by the arrow, the partition willprovide a shunt capacitive reactance. The magnitude of this reactancedepends upon the width of the aperture 4 in the direction of'theelectric field E and decreases as the width is decreased.

Fig. 2 is similar to Fig. 1 except that the aperture 4 extends from thetop to the bottom of the guide I and has its length parallel to thedirection of the electric field E. A partition of this type provides ashunt inductive rcactance the magnitude of which also decreases as thewidth of the aperture 4 decreases.

Fig. 3 is a perspective view, partly cut away, of a section of acircular wave guide I with a transverse partition 8 having a centralcircular aperture 9. This type of partition also provides a shuntinductive reactance which decreases as the diameter of the aperture 9 isdecreased.

By properly proportioning the aperture, a partition in a wave guide maybe made to provide both inductive and capacitive components in the rightamounts to resonate at a particular frequency. This may be either aparallel resonance or a series resonance. For example, Fig. 4 shows aparallel-resonant element, that is, one providwidth W perpendicularthereto. There are an infinite number of different apertures which willproduce parallel resonance,'but once either the height V or the width Whas been chosen, then the other dimension is thereby determined. Theline I; gives the locus of the upper right-hand corner l4 of allpossible rectangular apertures that, will provide parallel resonance inthe wave u de I.

Associated with each height V of the aperture l2 in the parallelresonant element shown in Fig. 4 there will be a resistance which iseflectivelyshunted across the guide I. The value of this resistancedecreases as the dimension V decreases and its range may extend from asmall fraction of the characteristic impedance of the guide Itoinfinity. It is possible, therefore, to design a Darticular resonantaperture which will have a shunt resistance equal to the characteristicimpedance of the guide. Such an element placed in the guide and followedby a solid metallic partition such as l5 placed one quarter of awave-length behind the element II will serve as a reflectionlesstermination for the guide I. A termination of this type uses noconventional resistance elements. The power is dissipated by highcirculating currents in the metal partition H which has high thermalconductivity and is in metallic contact with the walls of the guide Iand therefore is capable of dissipating a large amount of power. Theelement II, when used in a termination of the type described, ispreferably made of a metal having comparatively low electricalconductivity such. for example, as iron, since it is thereby possible tomake the aperture larger.

Fig. .5 shows a circular guide 1 having therein an impedance elementwhich may be adjusted for either parallel resonance or series resonance.The partition It has a rectangular aperture into which project a pair ofthreaded studs ll having their axes along a diameter of the guide I andparallel to the electric field E. The two internally threaded sleevesit, each with a circular metal plate is fast ned to one end, may bescrewed onto the s ds [1. The separation between the plates I! may thusbe adjusted as desired. For series resonance only a small separation isrequired. For parallel resonance the spacing will be greater, and inthis case the plates is may not be required. An advantage of using anaperture with one or more inwardly extending projections, as shown inFig. 5, is that sharper resonances may be obtained.

Fig. 6 shows an element more particularly adapted for series resonance,providing a low shunt impedance. The partition [6 has a symmetricalaperture 20 having its length perpendicular to the electric field E andits width constricted toward the center by. means of the inwardlyextending projections 2i and 22, to which are attached, on oppositesides of the partition i8, two overlapping metallic fiaps 23 and 24.These flaps 23 and 24 may be bent toward or away from each other toadjust the spacing therebetween and thereby the resonant frequency ofthe element. v

Fig. 7 shows a modification of the series-resonant element of Fig. 6 inwhich the flaps 23 and 24 are replaced by two opposing metallic plates2! and 26 which are perpendicular to the partition I8 and attached tothe ends of the projections 2| and 22.

Since a, metallic obstruction in a wave guide usually produces a pointof low potential and high current, it is preferable that the partitionbe secured to the walls of the guide by soldering, welding or in someother appropriate manner such that a good electrical contact isobtained. It should also be noted that thinner partitions than thoseshown in the drawings will. under some circumstances, produce moresatisfactory results. The partitions have been shown thicker in thedrawings only in the interest of clarity.

Fig. 8 shows how a variable shunt capacitive reactance may be providedin a wave guide I, which in this case is circular in cross section. Thetwo machine screws 30 and 3| enter the guide through holes on oppositesides and are disposed with their axes along a diameter and parallel tothe electric field E. Each screw threads into a nut, such as 32, whichis soldered to the guide in line with the hole. In order to provide agood electrical contact between the screw and the guide wall the nut 32is partially split longitudinally in one or more places, as shown at 33,and the resulting segments sprung inward to insure a tight fit. Thecapacitance may be increased by screwing the screws toward each other,or decreased by retracting them.

Fig. 9 is a perspective view, partly cut away. of a variable inductivereactor in a section of circular wave guide I. The screws 30 and 3| aresimilar to those shown in Fig. 8 but in this case their axes areperpendicular to the electric field E. Inside of the guide 1 is ametallic strip 35, made, for example, of spring brass or silver, whichis firmly attached to the guide at two opposite points by the screws 36.At two other opposite 'points the strip 35 has holes through which asmaller screw, such as 31, passes and threads into a tapped hole in theend of the larger screw 30'. When the screws 30 and 3| are retracted thestrip 35 lies against the wall of the guide However, as the screws 30and 3| are screwed toward each other the strip 35 is forced away fromthe guide wall at two places. There is thus provided a shunt inductancewhich decreases in value as the screws 30 and 3| are screwed in.

There will now be described some wave guide filters and transformerswhich use, as component parts, the reactive impedance elements describedabove. Fig. 10 is a perspective view, partly cut away, of asingle-chamber, adjustable band-pass filter in a rectangular guide Thefilter comprises two shunt reactors 38 and 39 spaced apart a distance Adetermined by the width of the transmission band desired and thewave-length x within the guide at the mid-band frequency. For narrowbands, A will be approximately equal to nA/Z, where n is any integer. Asthe band width is increased, however, the spacing A may departconsiderably from this value and, in fact, it will approach a value ofmA/4, where m is an odd integer.- 'To provide the greatestdiscrimination between the transmitted and the suppressed frequencies Ais made approximately equal to M2.

As illustrated, the reactors 38 and 39 are of the inductive type shownin Fig. 2, in which the slot in the partition is parallel to theelectric field E. In this case, for the greatest discrimination, thedistance A between the reactors must be made somewhat shorter than )./2.Alternatively, the reactors 38 and 39 may be of the capacitive type, asshown in Fig. 1, in which case, for the greatest discrimination, A mustbe slightly greater the reactor 30 may be a variable inductor of thetype shown in Fig. 9, in which case screwing the screws in willdecrease, and screwing them gut will increase, the effective length ofthe cham- The width of the band transmitted by the filter depends uponthe distance B between the two parts of the partition .38 and thedistance C between the two parts of the partition 39. The smaller thesedistances are made, the sharper will be the resonance and the narrowerwill be the band. If the filter is to be used to connect two sections ofguide having the same characteristic impedances, the spacings B and Care ordinarily made approximately, equal. In practice it is founddesirable to start by making the openings B and C somewhat undersized. Arough check of the frequency response will show that the resonance issharper than is desired. The openings are then enlarged in steps untilthe desired characteristic is attained. As the spacing is increased thetuning screws 30 and 3| are retracted slightly. When a very narrow bandis required, it will be found that an impedance match looking into thefilter from one direction will be obtained when the nearer aperture ismade somewhat larger than the farther aperture. For example, if the waveis entering from the left in Fig. 10, B is made slightly larger than Cin order to provide a characteristic impedance load for the sending end.

The guide and the partitions 38 and 39 of Fig. 10, as well as thecorresponding parts shown in-the other figures, may be made of brass orother alloy or metal of good electrical conductivity. The transmissionefilciency of the filters and transformers may be improved bysilver-plating the inner surfaces of the chambers.

The filter of Fig. 10 may be made impedance transforming, so that it canbe used to connect two wave guides having difierent characteristicimpedances, by making the opening into the higher impedance guide largerthan the opening into the lower impedance guide. For example, in Fig.10, if the right-hand termination has the higher impedance, the spacingC is made larger than B. By properly adjusting the spacing B, thepartition 39 may be entirely removed. This condition gives the widestpossible transmission band for any particular set of guide and chamberimpedances. The length A of the transformer section will, in general,depend upon the characteristic impedance of the guide and the impedancesof the reactors 38 and 39. However, the transmission band may be stillfurther widened by making the characteristic impedance of thetransformer section the geometric mean of the terminating impedances. Inthis case the partitions 33 and 39 may be reduced to flaps such as 65,55, 61 and 68 shown in Fig. 15 and described more fully below. Theseflaps perform the function of neutralizing the terminal reactances.

Fig. 11 is a perspective view, partly cut away, showing how two guides42 and 43 of unequal characteristic impedance may be connected togetherin a right angle without reflection. The guide 43, which has the lowercharacteristic impedance, extends beyond the junction and is closed by aslidable reflecting plate 44 which may be moved by means of the push rod46. The plate I is located at a distance from the mid-point of thejunction which, for bends in the electric plane, is equal approximatelyto A/2 and, for bends in the magnetic plane, is equal approximately toM4. The proper location of the plate 44 is the one which gives optimumtransmission and may be found by trial. There will, however, generallybe reflections of energy due to a mismatch of impedances at the junctionof the two guides. These reflections may be substan tially eliminated byadding a metallic flap 45 by means of which the opening D of thejunction aperture may be adjusted.

Fig. 12 is a perspective view, partly cut away, of a system fortransforming the impedance of a wave guide having a cylindrical sheath4'! and a solid concentric core 48 of dielectric material to match theimpedance of an air-fllled guide having a cylindrical sheath 49. Thecore 48 extends beyond the end of the sheath 41 for a distance F andextends into the sheath 49 a further distance G. Theintermediate-cylindrical metallic sheath 5. flts around the portion F ofthe core 45 and is conductlvely connected to the sheaths I! and 49 bymeans of the metallic end plates 5| and 52, respectively.

In order to match one wave guide to another one, or to any other wavemedium, it is, in general,

necessary to have two independent tuning controls. In the system shownin Fig. 12 these controls are the lengths F and G of the dielectric core45. The proper adjustment may be determined as follows. One of theguides is terminated in its characteristic impedance and wave energy issupplied to the transformer in such a way that it passes through astanding wave detector located in the other guide. Then the distances Fand G are adjusted, alternately, to minimize the standing wave. Thedesired adjustment is attained when the detector indicates an absence ofany standing wave.

A special case of the system of Fig.-12 is the one in which the sheath4! and the end plate 5| are omitted. This will generally require areadjustment of the distances F and G in order to get a proper impedancematch. The protruding portion of the core 48 may now be used as adielectric antenna for launching or collecting electromagnetic waveenergy.

Fig. 13 is a cross-sectional side view of a transformer for connecting aguide having a cylindrical sheath 55 fllled with a solid dielectric core55 to a guide having a cylindrical sheath 5! fllled with a material oflower dielectric constant such,

for example, as air. The sheath 55 and core 55 pass through the endplate 52 and extend into the sheath 5! for a distance H. The core 56alone extends beyond the sheath 55 for a further distance J. Thetransformer is tuned to transmit the desired mid-band frequency byalternately adjusting the distances H and J, as explained above, untilno standing wave is detected.

Fig. 14 is a cross-sectional side view showing an alternative form ofthe transformer of Fig. 13. The portion H of the sheath 55 internal tothe sheath 5'! has been omitted and the core 55 has an annular groove 58with an internal diameter L into which flts the end plate 52 to form ashunt impedance element. The core 55 extends into the ables in thistransformer are the distance K and.

the diameter L. These are adjusted, as already explained,.for nostanding wave.

Although Figs. 12, 13 and 14 show wave guide structures of circularcross section, it is to be understood that, with suitable modification,the transformers may be applied to rectangular or other forms of waveguides.

Fig. 15 is a perspective view, partly cut away, of what may be termed aneutralized quarter-wave transformer for connecting two wave guides IIand ii which diner in size and in characteristic impedance. The guides50 and I have rectangular cross sections of the same width M but diiferin the cross-sectional dimensions I1 and I: which are parallel to thedirection of the electric fleld E. The guides 80 and ii are connected byan intermediate section of guide 52 which has a length N approximatelyequal to a quarter wavelength, or an odd multiple thereof, at themidband frequency to be transmitted. The characteristic impedance of thesection 52 is made approximately the geometric mean of those of the gigdes 80 and II by making its height 1: equal to \/I1I:. Since the crosssection of the system is changed in the direction of the electric fleldE at each of the junction points 53 and 54, the junctions appear likeshunt capacitive reactances, of the type shown in Fig. 1. In order toneutralize these capacitive reactances the Junction II is constricted inthe magnetic direction by the addition of .the flaps 55 and and thejunction 54 is likewise constricted by the flaps 61 and 55. These flapsare made of proper width P to introduce a shunt inductive reactancewhich, at the midband frequency to be transmitted, is equal in magnitudebut opposite in sign to the associated capacitive reactance. In this wayeach junction 55 and M is converted into a parallel resonant shuntreactance of the type shown in Fig. 4.

Fig. 16 is a perspective view, partly cut away, of a band-pass waveguide filter comprising two resonant chambers 10 and II connected intandem. The cylindrical metallic sheath 12 has three partitions 13, 14and 15 with a spacing R equal approximately to a half wave-length, or anintegral multiple thereof, at the mid-band frequency to be transmitted.The partitions 13, ll and 15 have centrally located circular aperturesdesignated by their diameters S, T and U respectively. In order topermit an adjustment of the effective length R of the chamber, a pair ofoppositely disposed tuning screws I5 and I1 is provided for the chamber10 and a second similar pair 18 and II for the chamber H.

The fllter of Fig. 16 will, in general, have two peaks of transmission,the frequency separation between which will be decreased as the apertureT in the intermediate partition H is decreased in size. For asufliciently small aperture T the two pealm of transmission will fuseinto a single peak. As the aperture T is decreased in size it will benecessary to increase the eifective length R of each chamber by screwingin the tuning screws 15, i1, 18 and 19 in order to maintain the samemidband frequency. On the other hand, to broaden the transmission band,the aperture T is enlarged and the screws I6, 11, 18 and 19 areretracted.

After the desired separation between transmission peaks has beenobtained by an adjustment of the aperture T, as described above, thevalley between the peaks may be filled in, and thus a more uniformtransmission characteristic within the band provided, by increasing thesize of the apertures S and U in the end partitions I3 and I5,respectively. As the apertures S and U are increased in size, thechambers are retuned by retracting the tuning screws 16, 11, 18 and I9,in order to maintain the same mid-band frequency. Of course, theopposite adjustment may also be made. That is, the apertures S and U maybe decreased in size and the tuning screws screwed As long as the widthof the transmission band exceeds, say, one per cent of the mid-bandfrequency, the end apertures S and U are kept about the same size. Fornarrower bands, however, it will usually be found that a characteristicimpedance termination for the sending end may be obtained by making theaperture farthest away from the source of the wave energy smaller thanthe aperture nearest the source, For example, in the filter shown inFig. 16 if the waves enter from the left, the aperture U is made smallerthan the aperture S. At the same time the effective length R of thefirst chamber I is preferably made shorter than that of the secondchamber II. This adjustment is accomplished either by retracting thescrews I8 and 11 or by screwing in the screws I8 and I9.

It should be noted that the mid-band frequency of the transmission bandmay be moved in one direction or the other by adjusting the four tuningscrews. With the apertures S, T and U fixed in size, the mid-band may bemoved to a lower frequency by screwing in the screws I6, 11, I8 and I9,and it may be moved to a higher frequency by retracting all four of thescrews. To increase the height of one transmission peak and decrease theheight of the other transmission peak, the screws associated with onechamber, for example, I6 and 11, may be screwed in while the screws 18and 19, associated with the other chamber, are retracted.

Fig. 1'? shows a two-chamber filter similar to the one shown in Fig. 16except the variable reactors are of the inductive type shown in Fig. 9.The apertures in the partitions I3, I4 and I5 may be made larger orsmaller, as explained in connection with Fig. 16, for the same purposes.In this case, however, to adjust the effective lengths of the chambersI0 and II the tuning screws are screwed in when in the filter of Fig. 16they would be retracted, and they are retracted when in Fig. 16 theywould be screwed in. The filter of Fig. 17 may be designed and adjustedto give substantially the same type of transmission characteristic asthat obtainable with the filter of Fig. 16.

By using three or more coupled chambers connected in tandem a filterwith three transmission peaks, a more uniform transmissioncharacteristic, and sharper cut-offs may be obtained. Fig. 18 is across-sectional view, partly diagrammatic, showing, as an example, athree-chamber filter comprising a cylindrical metallic sheath 8| withtwo end partitions 82 and 85 and two spaced intermediate partitions 83and 84 which divide the guide into two end chambers 86 and 88 and anintermediate chamber 81. The two end partitions 82 and 85 have centrallylocated circular apertures 89 and 92, respectively, which are ordinarilyof approximately the same size and larger than the ordinarilyequal-sized apertures 90 and 9| in the intermediate partitions 83 and84, re-

spectively. Also, the end chambers 86 and 88 will usually have equallengths X while the intermediate chambers, such as 81, will have asomewhat longer length Y As shown, the three chambers 86, 81 and 88 havethe shunt impedances Z1, Z2 and Z3, shown diagrammatically, connected atthe respective mid-points. These impedances Z1, Z2 and Z3 may, forexample, be of the type shown in Fig. 8 or Fig. 9 and are preferablymade variable so that the effective length of the associated chamber maybe properly adjusted in the manner already explained.

The following adjustment procedure is suggested for the three-chamberfilter of Fig. 18. The end chambers 86 and 88 are given a length X ofapproximately a half wave-length, or an integral multiple thereof, atthe mid-band frequency to be transmitted and are individually tuned bymeans of the variable reactances Z1 and Z3 so that the primarytransmission peak will occur at the desired mid-band freqeuncy. The endchambers 86 and 88 are then assembled on either side of the centralchamber 81 which, for a threepeak filter, is given a length Y ofapproximately a half wave-length. or an integral multiple thereof, atthe mid-band frequency. The effective length of the central chamber 81is then tuned by means of the variable reactance Z2 until the twosecondary transmission peaks are spaced at equal distances on eitherside of the primary peak. Next, the apertures 90 and 9| in theintermediate partitions 83 and 84 are adjusted in unison to give thedesired band width. Finally, the apertures 89 and 92 in the endpartitions 82 and are adjusted in unison to produce a flat band.

The filter of Fig. 18 may be given a two-peak characteristic by makingthe length Y of the central chamber approximately equal to an oddintegral multiple of a quarter wave-length at the mid-band fre uency.This relegates one secondary peak nearly to zero or infinite frequencyand brings the other secondary peak nearly into coincidence with theprimary peak. By a proper adjustment of Z: these two last-mentionedpeaks may be separated by the required amount to give the desired bandwidth. All four of the apertures 89, 90, 9| and 92 are then adjusted toobtain a uniform transmission characteristic within the band.

Fig. 19 is a perspective view, partly cut away, of a band-suppressionfilter comprising a rectangular wave guide 96 and three tunedside-branch chambers 91, 98 and 99. The chambers are closed at'theirouter ends by the end plates I00, IIII and I02, respectively, and openinto the guide 98 through the apertures I93, I04 and I05. The centers ofthe apertures I03, I04 and I05 are spaced from each other approximatelya quarter of a wave-length. or an odd inte ral multiple thereof, at themid-frequency of the band to be suppressed. As in the other figures theelectric field E of the dominant transverse electric waves is polarizedin the direction indicated by the arrow. Each of the branch chambers 91,98 and 99 is tuned to resonate at the mid-band frequency by properlychoosing its length. and the resonance is made as sharp as desired by aproper choice of the width of the associated aperture I93, IM or I05,The three-branch filter shown may be designed to have high attenuationat the mid-band frequency and, on each side thereof, a frequency ofsubstantially perfect-transmission, giving very sharp cut-ofis.

It will be understood, of course, that either more or less than. threeside-branch chambers 11 may be used. Furthermore, the chambers maybranch from any of the four sides of the wave guide 96, although it willusually be preferable to place them along the sides which are parallelto the electric field E, as shown. The chambers may be tuned todifferent resonant frequencies to increase the width of the suppressionband. For example, two chambers, tuned to slightly differentfrequencies, may be used to provide two peaks of attenuation withsustained attenuation between. If a still wider band is desired, any oneor all of the branches I09, NH and I02 may be replaced by side branchesof the type shown in Fig. 21, described below.-

Fig. 20 shows another form of band-suppression filter comprising aside-branch chamber H0, opening into the guide 96 through the apertureI06, and a second chamber I01, coupled to the chamber H through theaperture I08 in the partition I09. Each of the chambers I01 and I I0 istuned to resonate at the mid-band frequency. The filter will have twoattenuation peaks the spacing between which depends upon the size of theaperture I08.

Fig. 21 shows a wave guide filter using a modified form of side branch-I H which may be designed either to transmit or to suppress a narrowband of frequencies. The branch H4 comprises an end chamber HI openingthrough an aperture H2 into a side-branch section I i3 of length Q1which connects the chamber III with the main wave guide 95: Shuntedacross the section I If at a distance Q2 from the side of the main guide96 is a reactive impedance branch Z4 which may. for example, be of thetype shown in Fig. 8 or Fig. 9. As already mentioned in connection withFig. 19. two or more branches H4 may be used to provide a wider band.

The adjustment of the filter of Fig. 21 is as follows. First, the endchamber Hi is tuned to resonate at the desired mid-band frequency. Then,for a band-pass characteristic, the length Q1 of the section I i3 isadjusted until waves of the midband frequency travelling through themain guide 95 are freely transmitted. The distance Q: is determined byfinding experimentally a point of standing wave voltage minimum withinthe section H3. The frequency of the waves is now changed to a frequencyconsiderably to one side of the mid-band and the magnitude of thereactance Z4 adjusted to produce a peak of attenuation. If a symmetricalcharacteristic is desired,

the value of Z4 is found first for a frequency ata certain distance toone side of the mid-band and then for a second frequency the samedistance to the other side of the mid-band. The reactance Z4 is then setat the average of the two values thus determined. For a band-suppressioncharacteristic the adjustment is the same as just described except thatthe length Q1 is adjusted for reflection of power at the midbandfrequency, and Z4 is adjusted for a transmission peak at a frequency toone side or the other of the mid-band.

Fig. 22 is a perspective view, partly cut away, of a branching filterarrangement for separating wave energy into individual channels on afrequency basis. The arrangement comprises a main rectangular wave guideH and five filters Hi, H1, H8, H9 and I20 each of which is connected tothe guide H5 through the front aperture. As shown, the filters are ofthe two-chamber type shown in Figs. 16 and 17 but are of rectangularcross section instead of circular. In the interest of simplicity thevariable reactances Q' of the electric field of the sociated with thchambers are not shown. It will be understood, of course. that eachfilter may comprise only two chambers. 1 The filters H6 to I20 are ofthe band-pass type, with different bid-band frequencies f1, is, Is, f4and f5, respectively. The corresponding wave-lengths at the mid-bandfrequency are M, in, A3, A4 and As, respectively. Each filter isdesigned so that, at its mid-frequency, it matches the guide 5 incharacteristic impedance.

One of the filters, H 5, is shown connected to the end of the guide H5.Alternatively, the end of the guide H5 may be closed by a metal plate.In order to terminate properly the main guide H5 over the frequencyrange for all of the channels, each filter, with the exception of I I,should be connected to the main guide at a point of voltage maximum forthe standing wave of the midband frequency of that particular filter.For example, the distances J1, J2, J: and J4 may be made equal to An,/05, AA: and %M. respectively. Now. assuming that the energy enteringthe guide 5, as indicated by the arrow l2], includes frequencies fallingwithin all of the bands, it will be separated by the filters Hi to I20into five individual channels, as indicated by the outgoing arrows. Ifthe mid-band frequencies h to Is have sufiicient separation, no filterwill be appreciably affected by the presence of the other filters.

Part of the subject-matter disclosed herein is being claimed in mycopending United States patent applications having the following serialnumbers and filing dates: 610,956 and 610,957, filed August 17, 1945;612,680 and 612,681, filed August 25, 1945, and 614,935 September 7,1945.

What is claimed is:

1. A filter for transmitting a band of guided electromagnetic wavescomprising a' metallic sheath, two spaced shunt reactors within saidsheath, and a-third shunt reactor within said sheath at a pointintermediate to said two reactors, said third reactor comprising a pairof opposed screws extending through said sheath.

2. A filter in accordance with claim 1 in which each of saidfirst-mentioned two reactors comprises a transverse partition with anaperture therein, said apertures being dissimilar.

3. A filter in accordance with claim 1 in which one of said reactorsconsists of means for restricting the cross-sectional area of saidsheath only in a direction 'pe pendicular to the direction waves to betransmitted. 1

4. A filter in accordance with claim 1 in which one of said reactorscomprises a transverse partition with an unsymmetrical aperture therein,the longest dimension of said aperture being substantially parallel tothe direction or the electric field of the waves to be transmitted.

5. A filter in accordance with claim 1 in which one of said reactorscomprises a transverse partition having an aperture which extends fromone side of said sheath to the other in a direction parallel to thedirection of the electric field of the wavesv to be transmitted.

6. A filter in accordance with claim 1 in which the axes of said screwsare in line and are substantially parallel to the direction of theelectric field of the waves to be transmitted.

'7. A filter in accordance with claim 1 in which the axes of said screwsare in line and are suba single chamber, or more than to 614,937,inclusive, filed ass 2.00s

13 stantially perpendicular-to the direction of the electric field ofthe waves to be transmitted.

8. A filt'er in accordance with claim 1 in which said third reactorincludes ametallic strip extending around said sheath on'the inside, theinner ends of said screws making physical contact with said strip andthe axesof said screws being substantially perpendicular to thedirection of the electric field of the wavesto be transmitted.

'9. A filter for transmitting a band of guided electromagnetic waves.-comprising a metallic sheath. two transverse apertured partitions spacedapart within said sheathv to form a chamber, and means for adjusting theffective electrical length of said chamber comprising a pair ofoppositely disposed screws extending through the walls of said sheathinto said chamber.

10. A filter in accordance with claim 9 in which the axes of said screwsare inline and are substantially parallel to the direction of theelectric field of the waves to be transmitted.

11. A filter in accordance with claim 9 in which said adjusting meansinclude a metallic strip extending around said sheath on the inside, theinner ends of said screws making physical contact with said strip andthe axes of said screws being substantially perpendicular to thedirection of the electric field of the waves to be transmitted.

12. A filter for transmitting a band of guided electromagnetic wavescomprising a metallic sheath and two transverse partitions thereinspaced apart a distance approximately equal to an integral multiple of ahalf wave-length for the mid-band frequency of said band, each of saidpartitions having an aperture therein and the areas of said aperturesbeing unequal.

13. In combination, a filter in accordance with claim 12 and a waveguide connected to one end thereof, the larger of said apertures beingthe nearer to said guide, whereby said filter is adapted to provide acharacteristic impedance termination for said guide.

14. A filter in accordance with claim 12 adapted to operate betweenunequal load impedances, the larger of said apertures being in thepartition nearer to the larger load impedance.

15. A filter in accordance with claim 12 which includes a variablereactor located within said sheath at a point intermediate to saidpartitions.

16. A filter for transmitting a band of guided electromagnetic wavescomprising a metallic sheath and three transverse partitions thereinforming two chambers resonant near the midband frequency, each of saidpartitions having an aperture therein and two of said aperturesdiffering in size.

1'7. A filter in accordance with claim 16 in which the apertures in thetwo end partitions differ in size.

18. A filter in accordance with claim 16 in which the intermediatepartition has the smallest aperture.

19. A filter in accordance with claim 16 in which the size of theaperture in the intermediate partition is so small that the filter hassubstantially a single peak of transmission.

20. In combination, a filter in accordance with claim 16 and a waveguide connected to one end thereof, the aperture in the partitionnearest to said one end being larger than the aperture in the partitionfarthest from said one end, whereby said filter is adapted to provide acharacteris- I, 14 21'. A filter in accordance with claim 16 in whichthe size of the aperturein the intermediate partition ls adjusted toprovide the filter with two transmission peaks having thedesiredfrequency separation .and the sizes of the other apertures areadjusted'to fill in the valley between said peaks and thereby provide asubstantially uniform transmission characteristic within said band. 22.A filter in accordance with claim 16 which includes a variable shuntreactor, within said sheath at a point intermediate to two of saidpartitions. 23. A filter in accordance with claim 16 which includesmeansfor adjusting the resonant frequencyof one of said chambers. a 24.A filter in accordance with claim 16 which includes means for adjustingthe resonant frequency of each of said chambers.

25. A variable inductive reactor for use in a wave guide comprising asheath, a metallic strip extending around said sheath on the inside andnormally lying in contact therewith, means for attaching said strip tosaid sheath at twoopposite points, and adjustable means at two otheropposite points for forcing said strip away from said sheath.

26. A reactor in accordance with claim in which said two points ofattachment lie in a line 7 screws.

29. A filter in accordance with claim 16 which includes a pair ofopposed screws extending through said sheath into one of said chambers.

30. A filter in accordance with claim 16 which includes a pair ofopposed screws extending through said sheath into one of said chambers,

the axes of said screws being substantially parallel to the direction ofthe electric field of the waves to be transmitted.

31. A filter in accordance with claim 16 which includes a pair ofopposed screws extending through said sheath into one of said chambers,the axes of said screws being substantially perpendicular to thedirection of the electric field of the waves to be transmitted.

32. A filter in accordance with claim 16 in which one of said chambershas therein a metallic strip extending around said sheath on the insideand means for adjusting the separation between said strip and saidsheath at two points which lie ona line substantially perpendicular tothe direction of the electric field of the waves to be transmitted.

33. A filter in accordance with claim 16 in which said chambers aretuned to different frequencies.

34. A filter in accordance with claim 16 in which the apertures in thetwo end partitions differ in size and said chambers have differenteffective electrical lengths.

35.-A variable inductive reactor for use in a wave guide comprising asheath, a metallic strip extending around said sheath on the inside andmeans for adjusting. the separation between said strip and said sheathat two opposite points which lie on a line substantially perpendicularto the direction of the electric field of the waves to be tic impedancetermination for said wave guide. 16 transmitted.

36. A variable reactor in accordance with claim 35 in which said meanscomprise a pair 01' screws extending through said sheath. t V

37. A filter for transmitting a band of guided electromagnetic wavescomprising a chamber, openings at opposite ends oi said chamber andmeans for adjusting the effective electrical length of said chambercomprising a pair of opposed screws extending through the walls of saidchamber.

38. A filter in accordance with claim 37 in which the axes oi saidscrews are substantially parallel to the direction 01' the electricfield of the waves to be transmitted. I

39. A filter in accordance with claim 37 in which one of said openingsis unsymmetrical and its longest dimension is substantially parallel tothe direction oi. the electric field oi the waves to be transmitted.

ARTHUR GARDNER 'mx. 9

summons crran tile oi this patent:

, UNITED STATES PATENTS Number Name Date 2,106,768 Bouthworth Feb. 1,1938 2,151,157 Schelkunofl Mar. 21, 1939 2,155,508 'Schelkunofl Apr. 25,1939 2,197,122 Bowen Apr. 16, 1940 2,253,503 Bowen Aug. 28, 19412,253,589 southworth A118. 26, 1941 2,323,201 Carter June 29, 19432,200,023 Dallenbach May 7, 1940 2,259,690 Hansen Oct. 21, 19412,406,402 Ring Aug. 27 1946 FOREIGN PATENTS Number Country Date 116,110Australia Nov. 4, 1942

